The use of rotating electric are for spherical particle productionW loan Bica *

This work presents an experimental device designed to obtain spherical partióles by means of a rotating electric are. A rotation frequency of the electric are of 750 s" , a voltage of 50 V (de) and a current of 100 A was used. The mass ñow rate was 3 g-min~ . Under these conditions particles of 15 to 20 JAVCI in diameter were obtained


INTRODUCTION
Metallic fine particles obtained by extraction or by flotation and those produced by milling recycled materials (e.g.chamotte, glass, etc.) exhibit irregular shape.In contrast, electric are or plasmabased technology^^ ^^ for metal surface covering, magnetofluidic composite materials^ ' and electrorheological fluids^"^^ production, etc. require spherical partióles^ \ On the other hand, for environment protection reasons, recyclable materials are of interest.For this purpose, we propose an experimental installation for sphere reshaping of powders resulting from the milling of chamotte.In addition, the resulting spherical particles may exhibit controUed coloration which is of interest in decoration and in the painting industry.

EXPERIMENTAL PROCEDURE
The scheme of the experimental device designed to produce spherical particles using a rotating electric are is given in figure 1.A general view of the device is presented in figure 2. The device consists of the current source: 1, the command panel; 2, the Figura 1. Esquema del bloque del modelo funcional destinado a la producción de partículas esféricas finas mediante arco eléctrico rotatorio; 1: fuente de energía, 2: pupitre de mando, 3 y 4: electrodos de cobre, 5: fuente de energía para la alimentación de las bobinas Ll y 12, ó: dosificador de polvo, 7: bombonas para el gas comprimado (argón) con reductores de presión y rotámetros, 8: medidor óptico, 9: bloque indicador de rotaciones; a y b: bornes de conexión, d: distancia entre electrodos.electrodes; 3 and 4, the source; 5 the supply of de power to the coils Lj and L2, the powder doser 6, the compressed gas cylinders, the optical sensor 8 and the rotation indicator block 9.The de current source is needed to power the electrical discharge between the electrodes.This equipment was obtained by modifying a WIG source, of type TED 400®.The technical characteristics of the current source are the foUowing:

•7H «óH
-supply voltage: 3 X 380 V, de, ± 10 %; -idle voltage: U,^.^, = 110 V, de, ± 10 %; -intensity of the continuously adjustable current in the range of (25 A, de, -200 A, de) ±10 %; The command panel consists of command and inter-blocking blocks for the current source, measure and control devices, high voltage and high Figure3.General view of the electrodes between which the electric are was primed; 1: coils, 2: well cooled electrodes.frequency oscillator for automatically priming the electric are, and a board with termináis for the connecting electrodes (Fig. 1).These electrodes are made of copper, and have a diameter of 80 mm.They are placed in parallel positions.The electric are is primed between the electrodes (Fig. 3).
When a magnetic field is applied perpendicularly to the electric discharge, the electric are begins a circular movement on the surface of the electrodes (Fig. 4)- The coils Li and L2 in figure 3 and figure 4, respectively, produce a resultant magnetic field perpendicular to electrical discharge axis.The máximum valué of the resultant magnetic field induction is B = 30 mT, for an electric current intensity supplied by source 5 (Fig. 1) of 28 A, de.
The powder doser (Fig. 5) is designed to mix the powder with the gas and to transport it into the electrical discharge.The mass flow rate of the powder can be adjusted between 0.

RESULTS AND DISCUSSION
The coils Li and L2 are connected to the de power supply 4 in figure 5 in such a manner that^ the resLilting magnetic field of induction, B, is perpendicular to the symmetry axis x-x^ (Fig. 6).The magnetic induction B, at the edge of the electrodes is plotted versus the electric current intensity I|, through the coils in figure 7.In order to bring the electric are into rotation, a forcé is needed, where á is the distance between the electrodes, r| is the dynamic viscosity of the médium in which the electric are moves, v is the tangent velocity, y is the superficial strain of the melt-gas system, r is the radius of the anodic spot, and, finally, 8 is the curvature radius of the separation surface of the melt-gas system.
While working, superficial melting of the copper electrodes occurs (Fig. 8).By denoting with y the superficial stress in the melt-argon system, one obtains from equation ( 1), after simple calculation, the frequency V of the circular motion of the electric are: Because of the cooling of the electrodes, the máximum valué of the discharge electric current is í = 100 A, de, for a voltage valué on the are U = 50 V, de.
The mean mass temperature of the electric are column in argón médium is estimated to be T = 3,000 K^^l Observing the trace left on the electrodes by the electric are (Fig. 8), it is measured r = 6-10-^ m, R = 3.5-10"^ m, d = 4-10"^ m and 8 = 2.5-10""^ m.For T = 3,000 K, it can be obtained r| = 4-10-4 kg.j^-i.s-iand Y = 0.9 N-m\ Figure 9 displays the predicted by equation ( 2) and the experimental valúes of V versus the valúes of the magnetic field induction B.
Deviations up to ± 20 % between the measured and the theoretical valúes can be seen in figure 9.These deviations occur due to the lack of precisión of the rotation indicator block, and, also, due to the modification of the thermo-physical characteristics of the metal surface as a result of the heating of the electrodes.
The chamotte powder (Fig. 10), milled in a hall mili, has an equivalent máximum diameter of 30 jLim.The powder is introduced in the rotating electric are at a flow rate of 3 g-min' ± 10 %, (the argón flow rate is 5 Nl-min"^ ).At rotation valúes up to 250 s"^ of the electric are, the processing efficiency is low (5 ± 10 %).Starting at V = 300 s"^ the powder processing efficiency increases.At V = 750 s the efficiency is máximum, (| ) = 92 %.On the other hand, V = 300 s"\ the plasma of the electric are includes the entire space between the electrodes.At T = 3,000 K, the chamotte powder is in permanent contact with the thermal field of the plasma.Due to the surface tensión in the meltargón system, spherical drops are produced and the ir shape is preserved as they are rapidly cooled (Fig. 11).The variation in diameters (ranging from 15 to 20 jLim) is due to the dispersión of the raw material as well as to the inhomogeneous thermal field of the electric discharge.An optimal valué of     Figuro 10.Polvo de ''chomofo'' procesado mecánicomente.
3 g-min"^ for the mechanically milled particles is obtained.The processing efficiency suddenly decreases for flow rate valúes exceeding 3 g-min" .A certain amount of particles stick to the electrodes leading to are extinction.

CONCLUSIONS
-Rotating frequency of the electric are up to V = 1,000 s"^ ± 5 was obtained.-The particles of the chamotte powder, obtained by mechanical milling, are shaped as spheres in the plasma of the rotating electric are.-The chamotte powder (mass flow rate 3 g-min' ) is spheroidized with an efficiency ([) = 92 % in argón médium (10 Nl-min"^) for an electric current intensity of 100 A, de, through the electric are, and a voltage on the are of 50 V, de.-A better cooling of the electrodes and a double valué of the idle voltage of the current source, should allow the spheroidization of a larger amount of particles.

Figure 7 .Figuro 7 .
Figure 7. Dependence of the resultant magnetic field induction B on the electric current ¡ntensity through the coils t-Figuro 7. Vorioción de lo inducción B del compo mognético consiguiente, en función de lo intensidod l¿ de lo corriente eléctrico en los bobinos.

Figure 8 .
Figure 8. Traces of meit on the copper electrode surface.

Figure 9 .
Figure 9.The magnetic field induction B dependence of the electric are rotational frequency at discharge electric current / = 100 A, de, plasma gas (argón) flow rate D = 10 Nlmin"^.